This invention relates generally to passive air traffic control and collision warning systems which utilize interrogation signals transmitted from a ground SSR station and associated transponder reply messages transmitted by vehicles, such as aircraft, that are elicited by the SSR, for determining the ranges, azimuth angles, altitude and identity of one vehicle relative to another or to the ground station and, more particularly, is concerned with improvements to systems of this type, specifically the collision avoidance system described in applicants, U.S. Pat. No. 4,486,755.
The system shown in U.S. Pat. No. 4,486,755, the disclosure of which is hereby incorporated herein by reference, receives standardized interrogation signals, having the waveforms shown in FIG. 1, transmitted from a ground station at a frequency of 1030 MHz on a narrow rotating main beam and in the side lobes of the main beam. The standardized interrogation signal consists of three pulses each 0.8 .mu.sec. wide: a P1 pulse; a P2 pulse spaced 2.0 .mu.sec. from the P1 pulse; and a P3 pulse spaced from P1 by either 8.0 .mu.sec or 21.0 .mu.sec. The P1 and P3 pulses are transmitted by the main beam and also, unintentionally, by the main beam side lobes which, unless suppressed, may be sufficiently strong to interrogate nearby transponders, creating false replies. Referring to FIG. 2, the rotating directional antenna that has been employed over the past several years, many of which likely will still be in service for many years to come, produces a scanning beam 10 which is about 2.5.degree. to 3.0.degree. wide at its 3dB point and slightly wider at its suppression control point.
In most SSRs, a second, static antenna omnidirectionally broadcasts a side lobe suppression control pattern 12 containing P2-only pulses or P1-P2 pulse pairs, wherein the P2 pulse is synchronized with the P1 pulse in the main beam, at a significantly higher level than the main beam side lobes, the purpose of which is to prevent transponder replies to other than main beam interrogation pulses when such main beam pulses exceed the P2 pulse suppression signal level by a fixed amount. On some SSR control patterns only P2 pulses are transmitted, which combine with the P1 pulses of stronger main beam side lobes to create a P1-P2 suppression pair. More specifically, the radiated amplitude of P2 at the transponder is (1) equal to or greater than the signal amplitude of P1 from the greatest side lobe transmission of the antenna radiating P1 (i.e., the rotating main beam 10) and (2) at a level lower than 9dB below the radiated amplitude of P1 within the desired arc of interrogation. When main beam P1 pulse levels exceed P2 pulse levels, P3 is no longer suppressed and P1-P3 pulse pairs interrogate transponders that are in the main beam.
A P1-P3 pulse pair with a separation of 8.0 .mu.sec. between P1 and P3 transmitted on the main beam interrogates the identity (Mode A) of a transponder-equipped aircraft, and a separation of 21.0 .mu.sec. between P1 and P3 interrogates that aircraft's altitude (Mode C). A series of about twenty such P1-P3 pulse pairs, one-half of which typically are Mode A interrogations and the other half Mode C interrogations, is received at a transponder, within a beam's width, during each 360 degree scan of the rotating beam. During the period that the rotating beam is pointing at the transponder, that is, the time the beam takes to scan approximately 4.degree., known as the "beam-dwell" time, the transponder replies in accordance with the "un-suppressed" P1-P3 spacings of the interrogation message. Interlaced Mode A and Mode C interrogation messages, such as ACACAC, or AACAAC, are separated by intervals typically of about 2500 .mu.sec. but in the range between a minimum of about 2,000 .mu.sec. to a maximum of approximately 5,000 .mu.sec. The broad SLS pattern, being significantly stronger at all azimuths outside of the main beam skirt (approximately 14-16 dB down from the main beam peak), prevents interrogation pulse pairs from being received by a transponder unless they are in the sector defined by the 3.degree.-4.degree. width of the main beam.
Summarizing, a P1-P3 pulse pair transmitted on the SSR main beam will interrogate an airborne transponder, causing it to transmit mode A and mode C messages, only if the amplitude of the P1-P3 pulses received at the transponder exceeds the amplitude of any received associated P2 pulses. Each qualifying transponder within the 360.degree. scanning coverage of the SSR main beam transmits in response a reply message on a 1090 MHz radio frequency carrier back to the SSR, with a known delay, so that the reply message is propagated along the path of the main beam and thus its signal strength is increased by beam gain, and received by a 1090 MHz receiver at the SSR. Each such 1090 MHz transponder transmission, known as a "reply message" and depicted in FIG. 3, includes a pair of framing pulses F1 and F2 separated by 20.3 .mu.sec. which define the start and stop, respectively, of the message, between which thirteen information pulses (twelve of which are currently used) are spaced in increments of 1.45 .mu.sec. from the first framing pulse, each of which is 0.45 .mu.sec. wide and may or may not be present depending upon the content of the message transmitted in reply to the 1030 MHz interrogation signal. The format of the message contained between framing pulses F1 and F2 is similar for any one of 4,096 identity codes transmitted. The absence or presence of each of twelve information pulses establishes which code is transmitted on 1090 MHz in response to the reception of an interrogating P1-P3 pulse pair spaced by 8.0 .mu.sec.
Similarly, the format of the message contained between the framing pulses is the same for any one of the altitude codes, which do not use D.sub.1 pulses, each of which represents the altitude of the aircraft to within .+-.50 feet in 100-foot increments up to a maximum in excess of 125,000 feet. Thus, the structure of the reply message allows for the possibility of 4,096 different code groups, each representing one or more pieces of information such as identity or altitude of the responding aircraft. As previously mentioned, 1030 MHz P1 and P3 interrogation pulses separated by 8.0 .mu.sec. when decoded elicit a reply code group transmitted on 1090 MHz representing identity. Similarly, a P1-P3 spacing of 21.0 .mu.sec. elicits a reply code representing the altitude of a given aircraft. As assigned by ATC or other authorities such as the military, the identity code is set in by the pilot with a cockpit "digit switch", while the altitude code is automatically established by a barometric altimeter and an associated encoder. The identity code designations consist of four digits, each of which lies between 0 and 7, inclusive, and is determined by the sum of the pulse subscripts given in FIG. 3. The identity code of the aircraft may be 1543, for example, which is represented by the presence of A.sub.1 ; (B.sub.1 B.sub.4); C4; and (D.sub.1 D.sub.2) pulses. The transponder automatically continuously transmits this identity code in response to every received Mode A interrogation regardless of which radar is interrogating, the beam width of the interrogating radar, or whether it is a civil, military or European radar.
In a similar manner, in response to interrogation P1-P3 pulses spaced by 21.0 .mu.sec., the transponder automatically looks at an automatic altitude encoder coupled to the aircraft's own barometric altimeter, which automatically changes the code with changes in altitude according to a pattern prescribed by the U.S. NATIONAL STANDARD FOR THE IFF MARK X (SIF) AIR TRAFFIC CONTROL SYSTEM (Oct. 10, 1968), and the reply message transmitted by the transponder is changed accordingly. Although the altitude information is presented in the same pulse format as the identity information, the ground system readily discriminates between Mode A and Mode C replies to its interrogation, because the relatively long interval between PRPs, and thus between interrogation messages, is such that only during a specific period of, say 3,000 .mu.sec., representing a round trip of about 250 nautical miles (3000/12 .mu.sec. per NM), following an interrogation message wherein the P3 pulse is spaced from P1 by 8.0 .mu.sec., all aircraft that are within the beam and within 250 NM respond with identity codes. Since the ranges of most SSR radars are limited to about 200 miles line-of-sight, all targets reply within typically 2500 to 3000 .mu.sec. During the following PRP, during which, say a Mode C interrogation is transmitted by the SSR, all aircraft out to a similar predetermined range that are intercepted by the main scanning beam will reply only with altitude codes. In this way there is no confusion between identity and altitude replies even though both use identical signal formats, because each pulse has a different significance. These identity and altitude codes are interpreted by an airborne collision warning system in the same way as does the ground station so as to provide collision warning data on all nearby transponders.
In the passive threat warning and collision avoidance system described in the '755 patent, an Own station receives interrogations from at least one and usually multiple SSR's within operating range, not only when the main SSR beam is pointing at it but also when Own station is illuminated by lower level side lobes of one or more main beams, and capitalizes on time of arrival (TOA) data from multiple SSRs to create a small cocoon of airspace that represents the approximate range and near exact altitude of any nearby transponder-equipped aircraft that may be a threat to Own's aircraft. Use of such transponders is mandated in some 240,000 aircraft in the United States alone and about 350,000 worldwide.
During a brief "listen-in" period of about 200 .mu.sec. initiated by Own's reception of a P1-P3 decode, Own station receives replies transmitted by transponders at Other stations in the general vicinity of Own station in response to each interrogation from an SSR. The received replies are decoded and using the P3 time of the associated interrogation message received by the transponder's 1030 MHz receiver, produce time of arrival (TOA) data for all surrounding aircraft and SSR stations within the sensitivity range of the Own station's 1030 MHz and 1090 MHz receivers. Operation of the '755 system depends on the amplitude of the side lobes of the rotating main beam being sufficiently high that a P1-P3 pulse pair would be received via the main beam side lobes so long as that receiver was within a given operating range of an SSR. Thus, the '755 system provides such TOA measurements not only during, but also before and after passage of the main beam, so long as P1-P3 pulse pairs can be received; the rotating main beam may be pointing in a direction other than at Own station and interrogating other transponders. Consequently, it is essential to the operation of the '755 system that it receive P1-P3 pulse pairs, and the associated 1090 MHz responses, both before and after passage of the SSR main beam through Own's station, throughout an angular sector of about .+-.30.degree. straddling the main beam's axis. The inability to receive P1-P3 pulse pairs in the deep nulls between the many such side lobes limited the effectiveness of the system.
The last decade has witnessed an evolutionary change in the design of ground SSR antennas, in particular the antenna system employed in SSR systems of the type here under discussion. Several hundred U.S.-based SSR's are now or are in the process of being equipped with an improved antenna system which is electrically phased so as to create a narrow, main scanning beam on which P1-P3 interrogation pulses are transmitted and reply messages are received, and which has very low side lobes. The new antennas usually do not include the static stand-alone antenna used in the earlier system for omnidirectionally broadcasting a P1-P2 side lobe suppression pattern, but, instead, employ antenna structure and radiating elements integral and rotatable with the rotating main beam-forming antenna structure for generating an SLS control pattern. As shown in FIG. 4, the SLS control pattern of this new system, containing either P1-P2 pulse pairs or only stand-alone P2 pulses, is generally "egg-shaped" in the horizontal plane, or may have a narrow null along the main beam's axis. The maximum signal of the SLS pattern, and therefore its maximum range of reception, is aligned with the axis of the main beam 16 and rotates with it; thus, the maximum signal level and therefore the range of the rotating SLS pattern traces an imaginary circle 18 as it rotates with the main beam. However, the signal strength is maximum only within a sector approximately .+-.40.degree. wide which straddles the rotating main beam. The signal level of the SLS control pattern in the direction of the main beam typically is about 14 dB to 16 dB down from the peak amplitude of the main beam and about 20 dB above the average level of the main beam side lobes. The level of the control pattern above the side lobe level varies with the angular displacement from the main beam, as much as 30 dB at an angle of 180.degree. from the main beam, while averaging approximately 20 dB above the main beam side lobes during a rotation period. The new SLS pattern exhibits high signal levels, without deep nulls, at all azimuths, within the .+-.40.degree. angular sector straddling the main beam, outside of which there is some diminution in level but still greatly exceeding the level of the main beams side lobes.
Unfortunately, however, this recent reduction in level of the main beam's side lobes turns out to be a disadvantage to the '755 system, the operation of which depends on reception of P1-P3 pulse pairs, not only those contained in the main beam but also those transmitted in and between adjacent side lobes. Consequently, the major reduction in side lobe level provided by the improved SSR antenna significantly reduces the operational range of the '755 system and, indirectly, the accuracy of its collision warnings, by reducing the probability of receiving multiple SSR's at most locations. As the population of improved antenna systems becomes larger, the useful collision warning range of '755 systems could be reduced.
Adding to the challenge is the fact that of the approximately 3,000 SSR's currently in service throughout the world, some already are using the improved antenna system, others are in the process of being updated, and others may continue using the "old" system, without change, for many more years. It is projected that there will be a "mix" of old and new antenna systems for approximately ten to twenty years before the "old" antennas are totally phased out.
Thus, there is a current and compelling need for a passive threat warning and collision avoidance system that is adaptable to the radiation characteristics of both the "old" and the "new" SSR antenna systems. The system should also be operable in geographical areas where only the P2 pulse is transmitted in the SLS control pattern, as is the case of SSR's in England and some other European countries. Some U.S. stations such as ASR-9 SSRs may also transmit only P2 pulses on the SLS control pattern.
Accordingly, the primary object of the present invention is to provide an "adaptive" collision avoidance system embodying the principles of the '755 system and capable of operation with any of the three types of ground radar transmission systems described above.
Another object of the invention is to extend the useful range of such system from an SSR thereby to increase the probability that SLS signals from two or more SSRs interrogating nearby targets will be received by the collision avoidance system and thereby significantly reduce false alarms and provide more precise measurement of pseudo-range.
Additionally, the system must be passive (that is, it should not itself transmit for the purpose of detecting a potentially colliding airplane), thus avoiding interference on either the 1030 MHz channel or the 1090 MHz channel of the standardized SSR system.
The system should also be relatively simple and inexpensive to manufacture so as to be economically feasible for owners of light aircraft such as those used in general aviation.